International Conference on Mechanical, Industrial and Energy Engineering 2014 26-27 December, 2014, Khulna, BANGLADESH
ICMIEE-PI-140119
Implementation of a waste-heat driven vapor absorption-based car air conditioning system
Md. Hasib Hossen 1,*, Md. Towhidur Rahman 2
1 B.Sc. in Mechanical Engineering, Khulna University of Engineering & Technology, Khulna-9203, BANGLADESH
2 B.Sc. in Mechanical Engineering, Khulna University of Engineering & Technology, Khulna-9203, BANGLADESH
ABSTRACT
Waste-heat driven air conditioning system eliminates compressor and introduces absorber, pump, and generator. Thus, the consumption of electricity is reduced and waste-heat of exhaust gas is used as energy input. The most common refrigerant-absorbent pair in vapor absorption system is NH3-water due to its availability, low cost, and high refrigerating effect. The drawback of using NH3 as refrigerant is its toxicity, corrosiveness, and explosiveness. A small capacity vapor absorption system is first analyzed for its implementation in car air conditioning unit. Components are designed based on capacity 2 kW. The appropriate equations describing the working properties are specified. The cooling load, COP of the unit, variation of absorption rate with NH3 weight percentage, variation of evaporator outlet temperature are examined. The experimentally obtained COP is 0.24 which is less than the designed COP 0.58.
Absorber heat rejection increases with increasing absorber temperature.
Keywords: Vapor absorption system; Automobile air conditioning; Waste heat; COP.
1. Introduction
With the development of modern technology, the engineers have become more concerned with the further technological development of the existing systems and eliminate the drawbacks of those systems. This includes improvisation of performance and efficiency, reduction in cost, precision, energy savings, usage of renewable energy, and many more. In this context, some alternative systems of car air conditioning have been developed.
Today, almost all car air-conditioning systems are charged with R-134a. However, alternatives with lower GWP than R-134a are desirable. Some new systems are being developed in order to revitalize the use of ecologically safe refrigerants. For example, a system for car air-conditioning using CO2 as the refrigerant has been developed by Lorentzen and Pettersen (1993). The testing of a laboratory prototype has shown that CO2 is an acceptable refrigerant for car air-conditioners.
Due to the international attempt to find alternative energies, absorption refrigeration has become a prime system for many cooling applications. Where thermal energy is available the absorption refrigerator can very well substitute the vapor compression system. It is a well-known fact that a large amount of heat energy associated with the exhaust gases from an engine is wasted. A rough energy balance of the available energy in the combustion of fuel in a motor car engine shows that one third is converted into shaft work, one third is lost at the radiator and one third is wasted as heat at the exhaust system (Greene and Lucas, 1969). Even for a relative small car-engine, such as for the Nissan1400, 15 kW of heat energy can be utilized from the exhaust gas (Wang, 1997). This heat is enough to power an absorption refrigeration system to produce a refrigeration capacity of 5 kW.
This project is based on the construction of a car cooling system by using the heat of exhaust gas. Here, the prime vision is to use the waste energy of exhaust gas and to construct a cooling system by giving the heat input in the refrigeration cycle from that exhaust gas.
In the conventional cooling system of a car, usually vapor compression refrigeration cycle is used. Here, a certain amount of energy from engine crank shaft is used to run the compressor. On the other hand, a huge amount of heat energy is released to environment as waste heat. If that heat is used as energy input of cooling process, it is possible to save a huge amount of energy. Moreover, the use of vapor absorption refrigeration system instead of vapor compression system eliminates the compressor. As a result, load on engine decreases and efficiency of engine increases.
2. Working Procedure of a Single Effect Aqua- Ammonia Air Conditioning System of a Car
The Working principle is shown in fig.1
Fig.1.a) Block Diagram of Simplified Single-effect Aqua-Ammonia Absorption System
Fig.1.b) P-T diagram of Aqua-ammonia absorption cooling cycle.
A simple vapor absorption cycle eliminates compression.
It uses absorber, pump, generator, analyzer, and reflux condensed in combined form as a replacement of compressor. Here, we will use aqua-ammonia system, where ammonia is used as refrigerant and water is used as absorber.
The low pressure refrigerant from the evaporator is absorbed by the liquid solution in the absorber. The pump receives low pressure liquid from the absorber and elevates the pressure. After that, the high pressure liquid solution is delivered to the generator. In the generator, heat from a high temperature source drives off the vapor that had been absorbed by the solution.
The vapor refrigerant then enters the condenser.
Remaining liquid solution returns to the absorber through a throttling valve whose purpose is to provide a pressure drop to maintain the pressure difference between the generator and absorber. In condenser, the vapor is cooled and condensed. There the heat is rejected in the atmosphere. After that the condensed refrigerant is passed through the expansion valve where the pressure is released. Then the low pressure refrigerant enters the evaporator. In evaporator, the refrigerant is evaporated by using the heat extracted from the refrigerated space. Thus, the space to be refrigerated gets cooled at a temperature lower than atmospheric temperature.
The main concern is to work with the function of a generator in vapor absorption refrigeration system. We have previously discussed that generator needs work input in the form of heat energy to separate refrigerant by evaporating it from the liquid absorbent solution. We are going to provide this heat input from the exhaust gas of car engine. From the exhaust pipe, it is possible to get an average temperature of 300◦c to 400◦c. By using an air to water heat exchanger, this heat can be extracted from exhaust gas and delivered to the generator. Thus, a huge amount of waste heat from the exhaust gas can be utilized in useful purpose.
3. Design of Air Conditioning System of a Car Cooling load was calculated for a passenger car and according to cooling load calculation, the other portions of an air conditioning system are designed.
3.1 Calculation of cooling load of a Compact Car Overall heat transfer coefficient: [1]
Heat transfer through glass: (conduction) Heat transfer through body: (conduction)
Heat transfer due to solar radiation: [1]
Total sensible heat= Sum of all Sensible heats
Effective sensible heat=Sum of total sensible heat and heat gain due to occupancy
Heat gain due to infiltration: [1]
Effective latent heat=Sum of all latent heat, heat gain due to infiltration and heat gain due to occupancy Grand total Heat, Q= Sum of Effective sensible heat and Effective Latent Heat
Obtained cooling load using the equations stated above is 4.393 KW.
If air locks are used in doors & windows, we can minimize filtration & by pass air considerably.
By using heat non-conducting leather or Rexene cover inside the body & seats, this heat can be considerably reduced. So, we will consider the cooling capacity of 2 KW.
3.2 Basic Assumptions and Design Parameters To perform designing of equipment size and performance evaluation of a single-effect Aqua- ammonia absorption cooler basic assumptions are made.
The basic assumptions are:
1. The steady state refrigerant is pure water.
2. There are no pressure changes except through the flow restrictors and the pump.
3. At points 1, 4, 8 and 11, there is only saturated liquid.
4. At point 10 there is only saturated vapor.
5. The pump is isentropic.
6. There are no jacket heat losses.
7. The capacity of the system is 2kW.
Some design parameters have been set up as per design requirement. They are stated below:
3.3 Evaporator analysis
Since, in the evaporator, the refrigerant is saturated water vapor and temperature (T10) is assumed 10 0C, the saturation pressure and enthalpy are calculated from curve fits [5].
Pressure and Enthalpy are determined by using these equations: [6]
P = 0.000000000002T6 – 0.000000003T5+ 0.0000002T4 +0.00003T3 + 0.0014T2 + 0.0444T
h = - 0.00125397T2 + 1.88060937T + 2500.559
Aqua-ammonia solution and refrigerant pressure and temperatures are obtained from these equations: [6]
Now, and are obtained from following equations:
ΣA = A0 X0 + A1 X1 + A2 X2 + A3 X3 ΣB = B0 X0 + B1 X1 + B2 X2 + B3 X3 For range 45% < X < 70% NH3 (Mass fraction, X = 60%) A0 = - 2.00755; A1 = 0.16976;
A2 = - 0.003133362; A3 = 0.0000197668 B0 = 124.937; B1 = - 7.71649;
B2 = 0.152286; B3 = -0.0007959
Enthalpy of NH3-water solution is obtained from following solution: [7]
Now, ΣB, and ΣC are obtained from following equations:
ΣA = A0 X0 + A1 X1 + A2 X2 + A3 X3 + A4 X4 ΣB = B0 X0 + B1 X1 + B2 X2 + B3 X3 + B4 X4 ΣC = C1 X1 + C2 X2 + C3 X3 + C4 X4
Where, these values are obtained for specific range of mass fraction:
A0 = - 2024.33; A1 = 163.309;
A2 = -4.88161; A3 = 0.06302948;
A4 = -0.0002913704.
B0 = 18.2829; B1 = -1.1691757; B2 = 0.03248041;
B3 = - 0.0004034184, B4 = 0.0000018520569.
C0 = -0.037008214; C1 = 0.03248041;
C2 = -0.000081313015; C3 = 0.00000099116628;
C4 = -0.0000000044441207
Mass balance on evaporator is as follows:
m9 = m10 + m11
Energy balance on evaporator is as follows:
Qe = m10 h10 + m11h11 – m9h9
From these two equations the value of m9, m10, and m11
can be obtained.
3.4 Absorber analysis
Since the values of m10, m11 are known, mass balance around the absorber can be stated as bellows:
m1 = m10 + m11 + m6
x1m1 = x6m6
From these two equations the value of m1 and m6 can be obtained.
The temperature and pressure around the absorber can be calculated by using the following equations:
Here,
ΣA = A0 X0 + A1 X1 + A2 X2 + A3 X3 ΣB = B0 X0 + B1 X1 + B2 X2 + B3 X3 A0 = - 2.00755; A1 = 0.16976;
A2 = - 0.003133362; A3 = 0.0000197668 B0 = 124.937; B1 = - 7.71649;
B2 = 0.152286; B3 = - 0.0007959 C = 7.05; D = -1596.49; E = -104095.5 The equation of enthalpy is as bellows:
Here,
ΣA = A0 X0 + A1 X1 + A2 X2 + A3 X3 + A4 X4 ΣB = B0 X0 + B1 X1 + B2 X2 + B3 X3 + B4 X4 ΣC = C1 X1 + C2 X2 + C3 X3 + C4 X4
A0 = - 2024.33; A1 = 163.309;
A2 = -4.88161; A3 = 0.06302948; A4 = -0.0002913704.
Parameter Symbol Value
Capacity Qe 2 kW
Generator solution exit temperature
T4 90 0C Weak solution mass
fraction
X1 55% NH3
Strong solution mass fraction
X4 60% NH3
Generator vapor exit temperature
T7 85 0C
B0 = 18.2829; B1 = -1.1691757;
B2 = 0.03248041; B3 = - 0.0004034184;
B4 = 0.0000018520569.
C0 = -0.037008214; C1 = 0.03248041;
C2 = -0.000081313015; C3 = 0.00000099116628, C4 = -0.0000000044441207.
The enthalpy at point 5 is not known but can be determined from the energy balance.
For, T = 34.9 0C, X0 = 55/100 = 0.55, Density of NH3
solution is obtained as: [7]
= 1145.36 + 470.84X0 + 1374.79X02 – (0.333393 + 0.571749X0) (273 + T)
Since, all variables are known, the pump power can be calculated as:
Energy balance on the absorber is stated by the following equation:
3.5 Generator analysis
The heat input to the generator is determined from energy balance equations:
3.6 Condenser analysis
The condenser heat can be determined from an energy balance equation given below:
3.7 Coefficient of performance The COP is defined as follows:
According to the previous calculations Energy flows at various component of the system are shown in table below:
4. Design of Heat Exchangers
In single-pass heat exchangers, the temperature difference ΔT between the hot and the cold fluids is not constant but it varies with distance along the heat exchanger. In the heat transfer analysis, it is convenient to establish a mean temperature difference (ΔTm) between the hot and cold fluids such that the total heat transfer rate Q between the fluids can be determined from the following expression: [3]
Mean temperature difference is calculated by using the equation stated below:
The overall heat transfer coefficient (U) based on the outside surface of the tube is defined as [3]
1 U =
For the design of the heat exchangers, the cooling water inlet and outlet temperatures are assumed. The cooling water inlet temperature depends exclusively on the available source of water, which may be a cooling tower or a well.
4.1 Absorber Design
For this design, the solution film can flow downward either on horizontal or on vertical tubes. The design of the horizontal tubes for the absorber, although theoretically well studied, presents a great problem with the shell tightness because of the large length of welds.
In the case of this study the water vapor produced in the evaporator is absorbed in the flow of the NH3-water solution and is condensing on the heat exchanger tubes.
The design of the heat exchanger therefore requires values for heat and mass transfer coefficients.
The fig.2 shows a model of absorber that has been used in car air conditioning unit.
Fig.2: Schematic representation of an absorber
Description Symbol kW
Capacity Qe 2
Pump work W 0.067
Absorber heat rejected Qa 2.8
Heat input to the generator Qg 3.45
Condenser heat rejected Qc 2.62
Design parameters of an absorber are stated in table below:
The independent variables, which affect the problem, are solution mass flow rate, solution inlet concentration, and absorber pressure and wall temperature. The data are correlated with the introduction of the “absorption percentage (Ap), defined as:
Determination of the equilibrium concentration, Ceq
requires the solution of the following set of expressions:
A = -2.00755 + 0.16976 X - 3.133362 .001 X2+ 1.97668 0.00001 X 3
B = 321.128 - 19.322 X + 0.374382 X2- 2.0637 .001 X3
C = 6.21147 D = -2886.373
T΄ = (-2 E / [D + (D2– 4 E (C – log (P / 6894.8))].5) - 459.72
TW = (5 / 9)*(AT΄ + B - 32)
E = -337269.46
The above set of expressions requires an iterative type of solution to find Ceq, given TW and P. In the case of this study TW=31 °C and P=1227 Pa, therefore Ceq= 0.52 and from Eq. stated above, AP=62.5. AP is
correlated to the length of plate (L) by the expression:
Where,
An iterative solution gives, m=0.0292 kg/m-s corresponding to the area of 5.4 m length pipe.
The next step is to check the area of pipes needed to cool the solution to the required level.
Patnaik et al. (1993), suggest that Wilke’s correlation, valid for constant heat flux wall with progressively decreasing difference from isothermal wall outside the entrance region, can be used for the falling film.
It is assumed that the flow is fully developed in a wavy, laminar regime and that the bulk solution temperature profile is linear with respect to the transverse coordinate (Patnaik et al., 1993). Wilke’s correlation is:
The film thickness is given by:
And the solution Reynolds number (Res) for the tube is:
Res=4Γ/μ 4.2 Generator Design
The generator provides sensible heat and latent heat of vaporization. The sensible heat raises the inlet stream temperature up to the saturation temperature. This amount of heat, typical in practice, is 13% of the total heat required [6]. The heat of vaporization consists of the heat of vaporization of pure water and the latent heat of mixing of the liquid solution.
Design parameters for generator design are stated in table below:
5. Construction
After designing all components, the construction is done according to the figure below. Stainless Steel tubes and galvanized iron plates are used for constructing these components.
Parameter Value
Tube dimension Tube outside
diameter Do = 12.7 mm and
inside diameter Di
=11.7 mm Cooling water inlet temperature 25°C Cooling water outlet temperature 26°C Mass flow rate of cooling water
(m)
0.21 kg/s
Absorber load (Qa) 2.8kW
Solution cooling From 44.6°C
to 34.9°C
Absorber pressure 1.227kPa
Inlet solution mass flow rate 0.0117975 kg/s
Parameter Value
Tube dimension Tube outside diameter Do = 9.53 mm and inside diameter Di
=8.5 mm
Generator pressure 9.66kPa
Generator solution Entering: 60% NH3 at 65°C
Leaving: 55% NH3 at 90°C
Generator water vapor mass flow rate (m)
0.00107 kg/s
Generator load (Qg) 3.45kW
Fig.3: Schemetic Diagram of Vapor Absorption based Cooling System of a Car
6. Result and Conclusion
6.1 Inlet outlet temperature of variouscomponents Heat Exchangers Cooling/Heating water
temperature (oC) Entering Leaving
Generator - 59
Condenser - 44
Evaporator 30 25
Absorber 25 26
6.2 COP analysis
The equation for COP calculation is: [1]
COP=
So, COP= 0.24 which is less than the COP obtained in design before. The actual overall heat transfer coefficient of stainless steel tubes are less than the ideal heat transfer coefficient. This is because of rust on the upper surface of tubes. Another reason is the less concentration of ammonia in absorber than designed concentration.
6.3 Conclusion
1. A car cooling system which uses exhaust gas as heating source is designed based on some correlations and formulas involving vapor absorption cycle.
2. Each component is constructed with locally available materials.
3. COP of the cooling unit is measured and compared with design COP.
4. The COP lower than the designed COP.
5. Some performance test is done to evaluate performance of the system.
6. Cooling unit offers a better environment.
7. A Fair amount of waste energy is used for their operation.
NOMENCLATURE Qe : Capacity
SHGF : Solar Heat Gain Factor
ETD : Effective Temperature Difference T10 : Evaporator temperature, °C
T4 : Generator solution exit temperature, °C X1 : Weak solution mass fraction
X4 : Strong solution mass fraction T3 : Heat exchanger exit temperature, °C T7 : Generator vapor exit temperature, °C ρ : Density, kg/m3
m11 : Liquid carryover from evaporator P : Saturation pressure, kPa
h : Enthalpy W : Pump work, kJ
Qa : Absorber heat rejected, kJ Qg : Heat input to the generator, kJ Q : Heat transfer rate, kJ/sec ΔTm : Mean temperature difference A : Total heat transfer area, m2 U : Overall heat transfer coefficient,
W/m2-°C
: Logarithmic mean temperature difference Re : Reynolds number
Pr : Prandtl number δ : Film thickness
F : Correction Factor for Heat Exchanger REFERENCES
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